The invention relates to a process for the production of a clamping means for clamping a component having contact surfaces in a holder, especially for clamping moulding tool inserts in a support plate in order to produce a moulding tool for the manufacture of precision articles, for example contact lenses.
The invention further relates to a clamping means for clamping a component having contact surfaces in a holder, especially for clamping moulding tool inserts in a support plate in order to produce a moulding tool for the manufacture of precision articles, for example contact lenses.
With moulding tools used to manufacture precision articles, it is very important that the tool inserts are held in a releasable manner in the support plate in precisely defined and reproducible positions. Clamping means are used for this purpose. However, such clamping means may also be employed for example for the positioning and clamping of workpieces, for example during the clamping operation, in which high clamping precision of the workpiece is similarly required.
The invention further relates to a process for producing a moulding tool for the manufacture of precision articles, especially contact lenses, whereby the moulding tool has
two tool halves, each half containing a tool holder into each of which is clamped one tool insert of at least one pair of shape-determining tool inserts having contact surfaces, and
positioning means, by means of which the tool holders when the moulding tool is closed are positioned relative to one another in such a way that each of a pair of shape-determining tool inserts is opposite the other and interacts with the other to form a mould cavity.
The invention also relates to a moulding tool for the manufacture of precision articles, especially contact lenses, containing
(a) two tool halves, each half containing a tool holder into each of which is clamped one tool insert of at least one pair of shape-determining tool inserts having contact surfaces, and
(b) positioning means, by means of which the tool holders when the moulding tool is closed are positioned relative to one another in such a way that each of a pair of shape-determining tool inserts is opposite the other and interacts with the other to form a mould cavity
Moulding tools of this kind for the manufacture of contact lenses require very high precision of concentricity between two opposed tool inserts, whereby the eccentricity of the closed moulding tool may not be greater than xc2x15 xcexcm. This in turn leads to very high precision requirements of the clamping means used. In addition, the clamping power of the clamping means must be sufficient for an axial force of  greater than 500 N to be capable of being exerted on the tool insert without any shift of the clamped tool insert in an axial direction.
Commercial clamping means for tool machines, in which a workpiece is positioned and clamped, only fulfill the precision requirement of  less than xc2x15 xcexcm to a limited extent in their standard programme.
Moulding tools for the manufacture of contact lenses are known. They consist of two halves of a mould, in which several pairs of opposed moulding tool inserts are arranged in two support plates. The tool inserts of the two mould halves are in alignment with one another and when the mould is closed they form a mould cavity between them. In known moulding tools for the manufacture of contact lenses, the support plates are made of aluminum. The moulding tool inserts are clamped in the support plates by clamping means. To serve this purpose, a thin-walled, cylindrical clamping bush is inserted in a bore of the support plate and is fastened to the support plate by a flange. A sleeve-shaped holder for the tool insert is seated in the clamping bush. The clamping bush forms a pocket, into which pressure means (clamping oil) can be introduced. As a result, the wall of the clamping bush is deformed and the holder is clamped in place. A centring pin serves to position the clamping bush initially. The support plates may hold several tool inserts, which form several mould cavities for simultaneous manufacture of several contact lenses or the like, in the manner described. With this known moulding tool, a centring precision and a concentricity of the clamped tool inserts of xc2x15 xcexcm is achieved.
These known moulding tools each require a large number of high-precision components. All the components require complicated surface treatment. The registration of the tool inserts opposed to one other in the two mould halves is difficult and time-consuming and hence expensive. Tolerances of the components are cumulative, so that precision in the known moulding tools is limited. For this reason, great care must be taken when handling the moulding tools. Furthermore, the hydraulic clamping means takes up a lot of room, so that only a relatively small number of tool inserts can be accommodated in a given support plate of the dimensions concerned.
Furthermore, with these known moulding tools, process steps involving the use of heat are not permitted. The linear expansion of the components is variable. As a result, when there are temperature changes, both the alignment and the clamping power are greatly affected. The oil pressure of the hydraulic clamping system also alters under the action of heat.
A problem arises when the mould is opened. Then, the moulded precision articles may be retained by either one or the other of the mould halves. This depends on random influences that are difficult to control. This uncertainty makes it difficult to remove the precision articles, especially if this is to be carried out automatically by a machine.
A moulding tool for the manufacture of precision articles, especially contact lenses, is known from DE 196 10 563 A1. This is intended to eliminate the above-described disadvantages. As with the above-described moulding tool, the moulding tool consists of two mould halves, in which several pairs of opposed shape-determining moulding tool inserts are arranged in bores in two support plates. No clamping means are used to clamp the tool inserts in the support plates, but the tool inserts are held in the bores of the support plates by optical wringing. The tool inserts of the two mould halves align with one another and when the mould is closed form a hollow cavity between them. Both the tool inserts and the support plates consist of a material which has negligible thermal expansion at the operational temperature range of the moulding tool. This material may be a glass ceramics material, or a specific metal alloy such as invar or quartz glass. In this way, the geometry of the bores of the support plates, which serve to receive the moulding tool inserts, does not change in dependence on temperature, so that the tool inserts maintain their positions relative to the support plates even during temperature changes.
A disadvantage of this moulding tool described in DE 196 10 563 A1 is the complex production process. The conventional tool inserts used in the above-described moulding tool cannot be used here. New tool inserts have to be produced, whereby the contact surfaces have to be worked very precisely, so that optical wringing is possible. Since the connection between the tool inserts and the support plates is effected by optical wringing, release of the tool inserts is complex.
The invention is based on the problem of providing a clamping means to clamp a component in a holder, which is of simple construction, and by means of which the component to be clamped can be clamped in the holder with high precision.
The invention is based especially on the problem of providing a moulding tool for the manufacture of precision articles, particularly contact lenses, in which tool inserts can be clamped in tool holders in a simple manner with high precision.
This problem is solved according to the invention by a process for the production of a clamping means according to claim 1 or a process for the production of a moulding tool according to claim 2.
This problem is also solved according to the invention by a clamping means according to claim 6 or a moulding tool according to claim 7.
The recess or recesses in the holder may be blind-end bores or continuous bores with cylindrical walls. In this case, the first contact surfaces of the clamping means may consist of an annular or cylindrical superficies.
The contact surfaces of the component or tool insert to be clamped may consist of a cylindrical superficies. In the case of a moulding tool for the manufacture of contact lenses according to the invention, the tool inserts to be clamped are preferably identical to the tool inserts that are used in the above-mentioned known moulding tool for the manufacture of contact lenses, so that it is not necessary to produce new types of tool inserts. The second contact surfaces of the clamping means in this case may be annular or cylindrical and may be provided directly at the spring-action clamping elements.
The recess or recesses in the holder may be graduated bores forming a ring shoulder. The spring-action clamping elements may be formed by one or more tightening discs having annular contact surfaces which in the clamped state of the component to be clamped adjoin a cylindrical superficies of the component to be clamped. A first tightening disc may adjoin the ring shoulder of the bore. In order to obtain a precisely defined position of this tightening disc, however, it is advantageous to provide a contact disc between the ring shoulder and this tightening disc, this contact disc forming a sharp edge with the wall of the bore. Any further tightening discs that may be provided can be arranged directly next to this first tightening disc. The tightening discs are operated mechanically. This can be effected by an actuating component in the form of an actuating sleeve, through which pressure is applied to the tightening discs, so that the tightening diameter of the tightening disc or discs is reduced.
Several tightening discs may also be provided at several levels of the recess, and are separated from one another by washers. In this way, the component to be clamped is clamped at several levels, thus preventing the clamped component from tilting in the holder.
However, the spring-action clamping elements may also be formed by one or more clamping sleeves having cylindrical contact surfaces, which in the clamped position of the component to be clamped make contact with a cylindrical superficies of the component to be clamped. To achieve the springing action of the spring-action clamping sleeve, the wall of the clamping sleeve may be provided with slots which permit the clamping diameter of the clamping sleeve to be changed. The clamping sleeve may have a conical outer face. A centring sleeve provided with a conical inner face designed to be complementary thereto may adjoin a ring shoulder of the bore. The clamping sleeve is operated mechanically, slipping into the centring sleeve, whereby the conical outer face of the clamping sleeve interacts with the conical inner face of the centring sleeve, so that the clamping sleeve is compressed around the component to be clamped, thereby firmly clamping it. This may be effected by an actuating component in the form of an actuating sleeve, whereby the actuating sleeve may be joined to the clamping sleeve. Since the clamping sleeve adjoins the component to be clamped with a relatively large contact area, the component to be clamped is also prevented from tilting when using a single clamping sleeve. In addition, good cohesion can be achieved, and the contact pressure at the tool insert is minimised.
To produce the clamping means according to the invention or the moulding tool according to the invention, firstly all the components are produced individually by customary manufacturing processes, maintaining the usual manufacturing tolerances of precision manufacturing, for example in the range of xc2x10.05 mm or even greater. If the components thus produced were to be constructed together for use as a clamping means or moulding tool without undertaking the working according to the invention of the second contact surfaces of the clamping means, there would be unacceptably large inaccuracies in centring or coaxiality of the parts or tool inserts to be clamped, since the tolerances of the individual components are cumulative. Production of the components whilst maintaining manufacturing tolerances, leading to satisfactory precision in the assembled state, is not possible from an economic point of view when using conventional manufacturing techniques.
The invention is based on the knowledge that tolerances of the individual components play a secondary role. The decisive factor is merely the tolerances occurring in the clamped state of the part or tool insert to be clamped between the contact surfaces of the clamping means and the contact surfaces of the part or tool insert to be clamped. In order to keep these tolerances low, the components produced by customary manufacturing processes are then assembled, so that the clamping means or the spring-action clamping elements are pre-tensioned to a specific degree, whereby the part or tool insert to be clamped is not yet used. The specified initial tension of the clamping elements is chosen so that the second contact surfaces of the clamping means are approximately in the position in which they later adjoin the contact surfaces of the part or tool insert to be clamped. In this position, the second contact surfaces are then prepared, for example by grinding.
To maintain this defined initial tension of the clamping means, a washer can be provided. The washer may be used in such a way that it serves as a stop for an actuating component to actuate the clamping elements. The washer is of a dimension such that the clamping elements are under the desired, defined initial tension when the actuating component adjoins the washer. In the moulding tool according to the invention, the tool holders preferably consist of support plates, whereby the recesses that receive the tool inserts extend through the support plate, so that it is possible to access the inserted clamping means from both sides of the support plate. The second contact surfaces of the clamping means can then be finished when the moulding tool is closed, the second contact surfaces of the clamping means inserted in each of the two aligned recesses preferably being finished together. This can take place by grinding through two recesses in one grinding process. A high concentricity between the second contact surface of the clamping means of two opposed recesses is thereby achieved.
A moulding tool according to the invention may contain an actuating component, through which the spring-action clamping element(s) of all clamping means associated with a tool holder are actuated simultaneously. This actuating component may be a plate, through which parts of the clamping means are pushed into the recesses when the plate is pressed against the tool holder. If the plate is then located in a defined layout on the tool holder and is secured there, these parts of the clamping means are located in precisely defined positions. This ensures that the clamping means are clamped in the same way in all recesses of the tool holder, so that the clamping power is equal.
In the clamping means according to the invention and the moulding tool according to the invention, the positioning precision of the component or tool insert to be clamped is only dependent on the manufacturing precision of the second contact surfaces of the clamping means, and not on the precision of the apparatus or registration thereof. This means that adjustment, which is generally very difficult and time-consuming, can be dispensed with.
The moulding tool according to the invention may be of very robust construction. There are no hydraulic clamping means. Handling of the moulding tools is thus simplified.
Through the simple and robust construction of the moulding tool according to the invention, the maintenance and cleaning operations are reduced, assembly of the equipment is simplified, the weight of the moulding tool and the tool costs are reduced, and a non-sensitive and safely-functioning moulding tool is created. In addition, thermal process steps can be carried out, and it is possible to clean the moulding tool with tempered water.
It has been shown that with a moulding tool according to the invention, both the centring precision and the concentricity of the clamped tool inserts and also the reproducibility of clamping are improved, whereby centring precision and concentricity of better than xc2x12.5 xcexcm can be attained. In addition, there is less disalignment of the clamped tool inserts upon repeated opening and closing of the moulding tool.
The clamping means according to the invention can be produced with substantially smaller dimensions than the hydraulic clamping means of the prior art. Thus, the tool inserts arranged in proximity in a tool holder can be considerably closer to each other, whereupon the number of tool inserts in a support plate of a specific dimension and thus the output of precision articles can be increased considerably.